1. Field of the Invention
The present invention is related to nonvolatile storage and more particularly to integrated circuit chips including nonvolatile storage such as one or more cells or an array of nonvolatile random access memory (NVRAM) cells.
2. Background Description
Semiconductor memories made in well known complementary, insulated-gate Field Effect Transistor (FET) technology, commonly referred to as CMOS, are well known in the art. A typical semiconductor memory is an array of memory cells arranged in rows and columns. Each cell is addressed by coincidence of a row with a column. When each row is selected, e.g., driving a word line, cells in the row are connected to the respective column or bit lines. So, cells may be accessed by selecting a row, and checking for a signal response on one or more columns.
In what is well known as a Read Only Memory (ROM), each ROM cell has fixed contents that may not be changed. A typical ROM cell, for example, is a transistor (FET) or a diode. ROM cells are personalized by either the presence or absence of a transistor or diode in each cell, or a connection to a cell transistor or diode. Before even low level integration, for example, ROMs routinely were assembled of an array of discrete diodes by selectively wiring together the anodes of selected diodes in a row, and wiring together the cathodes of diodes in each column.
More recently, however, circuit structures in semiconductor Integrated Circuits (ICs) are normally formed on the surface of a semiconductor substrate or, for Silicon-on-insulator (SOI), on the surface of a semiconductor (silicon) layer. Diodes have not proven very efficient for SOI ROM cells. Discrete diodes are impractical in bulk silicon because active devices, including diodes, share the bulk substrate in common, i.e., transistor collectors/FET substrates are in the same shared diode terminal (anode/cathode). Further for addressability, SOI diode ROM cells typically require FETs that consume additional space.
A typical SOI diode has contacts to both the anode and cathode on the same surface of a silicon surface layer. Several diodes may be formed in the surface layer, which acts as a shared common anode or cathode for the diodes, e.g., spaced in a silicon surface layer strip at sufficient distance to isolate them from each other. Consequently, a contact to that shared terminal (silicon surface layer anode/cathode), e.g., at one end or the other or the middle of the silicon strip, is invariably closer to some diodes than to others.
For a typical state of the art thin silicon surface layer, the sheet resistance (ρ) may be greater than one thousand ohms per square (1KΩ/). So, even before adding diode junctions, which act to increase ρ in the vicinity of each diffusion, the resistance of such a strip may be tens of KΩ. This resistance further increases as the line width for the strip narrows (for density) and/or the strip is lengthened (for increased capacity). Moreover, when the maximum available voltage is limited (currently to approximately 1 Volt), the voltage dropped by diode current flowing through this resistance can absorb a large percentage of the available signal. Conventional saliciding of the silcon strip to reduce resistance tends to short the surface electrodes together, frustrating individual diode formation. Consequently, a ROM designer is faced with either using wider, unsalicided strips of diodes for a much less denser array or, accepting signal loss from high resistance in return for density.
Thus, there is a need for a dense SOI ROM and, more particularly dense SOI ROM arrays on minimum pitch in both row and column directions.